The present invention relates to a semiconductor device having a capacitor in which a capacitor insulating film and an upper electrode are laminated to cover a convex shaped lower electrode, and also to a method for manufacturing the semiconductor device.
In recent years, as the semiconductor integrated circuit has been highly integrated, the minimum fabricating dimension and the area of a memory cell have become smaller and smaller. Accordingly, the area of the capacitor in a memory has been very small. Although the capacitance (storage capacitance Cs) is reduced in accordance with the reduction in area of the memory cell, it must be greater than a certain value from the viewpoint of the sensitivity, soft error, circuit noise, etc. To solve this problem, two methods have been considered: one is to form a three-dimensional capacitor in order to increase the area of the capacitor as large as possible with a small cell area, thereby increasing the capacitance; and the other is to use an insulating film having a high dielectric constant (so-called highly dielectric material) as the capacitor insulating film.
In a generation of the 0.15 μm or smaller design rule (in the generation of the 512 M bit DRAM or later generations), it is difficult to work a storage node (SN) electrode having a complex three-dimensional form, since a microfabrication process is required. Therefore, to increase the capacitance, it is very important that the capacitor have a three dimensional form and the capacitor insulating film be made of highly dielectric film.
A (Ba, Sr)TiO3 film (hereinafter referred to as a BST film) is a representative of highly dielectric films. Since an oxygen atmosphere is used during formation of the BST film, it has been proposed to use, as a storage node electrode, an Ru film which maintains conductivity even after oxidized in the process of forming the BST film (an RuO2 film has conductivity), or an RuO2/Ru laminated film (1995 IDEM Technical Digest, S. Yamamichi et al., pp. 119-122).
A structure of a stack-type DRAM capacitor in which a storage node electrode is made of an RuO2/Ru laminated film will be described with reference to FIG. 15. First, an element isolating region 12 is formed on a p-type Si substrate 11. Thereafter, a gate oxidized film 13 of a transistor, a gate electrode 14 which serves as a word line in a memory cell region, a gate cap layer 15, a source/drain region 16 and a silicon nitride film 17 are formed. After a first interlayer insulating film 191 is deposited and flattened, polysilicon plugs 20a and 20b are buried in a storage node electrode contact region and a bit line contact region, and then a bit line 26 is formed via a second interlayer insulating film 192. Subsequently, after a third interlayer insulating film 193 is deposited and flattened, an SN contact hole is opened and an n−-type polysilicon plug 194 is buried therein. Then, a film of a storage node electrode forming material is formed, and thereafter patterned by means of the conventional lithography and RIE using a resist film. As a result, a storage node electrode 27 is formed. After the resist film is removed, a capacitor insulating film 29 made of highly dielectric material, such as a BST film, is formed on the storage node electrode 27. Further, a plate electrode 30 is formed on the capacitor insulating film 29.
Since the storage node electrode is formed by etching the electrode forming material by the RIE, the side surface of the storage node electrode is damaged and in a bad surface condition. The BST film grows, succeeding to the crystal conditions of the storage node electrode on which the BST film is based. Therefore, the damage to the storage node electrode influences the growth of the crystal of the BST film, causing reduction in dielectric constant of the BST film, or results in distortion of the crystal of BST and increase in leak current of the BST film. Thus, the damage to the storage node electrode is a factor in limiting the decrease in thickness of the BST film.
Further, since the storage node electrode grows from the surface of the interlayer insulating films, it has a columnar crystal structure mainly along the vertical direction. Therefore, on the side surface of the storage node electrode constituting a main capacitor, the orientation of the crystal of the storage node electrode is mainly perpendicular to the BST film.
In addition, the storage node electrode structure described above has a problem that upper corners of the storage node electrode become acute and the electric field is concentrated in those portions, resulting in that a very large leak current is generated in the capacitor insulating film.
FIG. 16 shows an SrRuO3 electrode (storage node electrode) 202 formed over a convex shaped TEOS film 201 projecting on a Si substrate 11. The orientation of the crystal of a BST film (capacitor insulating film) 203 greatly varies in corner portions of the electrode 202. Therefore, the crystal is distorted in the portions, with the result that a very large leak current is generated in the capacitor insulating film. This is a restricting factor that hinders reduction in thickness of the capacitor insulating film.